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J. J. Hunter, C. J. Cookson, M. L. Kisilak, J. M. Bueno, M. C. W. Campbell; Metrics to Quantify the Global Quality of Confocal Scanning Laser Ophthalmoscopic Images of the Aging Eye. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2768.
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© ARVO (1962-2015); The Authors (2016-present)
Good quality images of the human fundus are critical for the diagnosis, treatment and monitoring of ocular disease. In order to validate fundus image quality metrics, we used a polymer dispersed liquid crystal cell to introduce scatter (expected to degrade image quality) into a confocal scanning laser ophthalmoscope during fundus imaging in participants of differing ages.
A CSLO was modified to include a liquid crystal scatter generator in the scanning path, conjugated with the pupil of the eye. Video segments of the retina, including optical sections through the optic nerve head were recorded for differing degrees of scatter and no scatter in 12 healthy adults aged 19-64 years. This was done for a series of confocal pinhole sizes. For each condition, frames were registered and averaged. For each averaged image, we calculated three global metrics of image quality (signal to noise ratio, entropy and acutance). A repeated-measures ANCOVA (with age as the covariate) was performed for each image quality metric.
For a single series of averaged CSLO images, SNR and entropy increased in relation to the number of images in the average. Conversely, although acutance is expected to increase with increasing image quality, it decreased as the number of averaged images increased. Induced scatter reduced quality over all pinholes, with less reduction with a small pinhole. The image quality metrics increased with increasing pinhole diameter, likely due to a loss in axial resolution. For entropy, there was a significant interaction among pinhole, lens and scatter. Across all conditions, SNR was poorly correlated with entropy (R=0.439) and acutance (R=0.378). Entropy and acutance were very highly correlated (R=0.940). This result differs from the change in metrics with frame averaging, where entropy and acutance were negatively correlated. The metrics of image quality had values that were greatest for images focused on the nerve fibre layer surrounding the optic nerve head and with no induced scatter. Entropy as a function of defocus was used to estimate the axial resolution of the CSLO.
Entropy appears to be the most useful objective, global metric for quantifying differences in fundus image quality. The image quality metrics are affected by the increase in image intensity at larger imaging pinholes resulting from reductions in axial sectioning capability.
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